Mycoplasma hyorhinis is an etiologic agent causing degenerative polyarthritis in its natural host, the swine. This disease displays characteristic features of other mycoplasmoses, including chronicity and autoimmune phenomena. Both of these characteristics may ultimately arise from the propensity of mycoplasmas to colonize surfaces of host cells, possibly resulting in the simultaneous stimulation of immunopathological reactions, and evasion of host-immune responses. An in vitro model has been developed to study the interaction of M. hyorhinis with murine T-lymphoid cells. Results from this model suggest that mycoplasmas 1) selectively acquire host cell surface antigens, 2) mediate a C-dependent humoral response in vitro, leading to destruction of host cells colonized by these organisms, and 3) induce rearrangements of T-lymphoblastoid surface antigens. All of these phenomena could profoundly affect the interaction of mycoplasmas with the host immunological apparatus in vivo. Surface membrane constituents of mycoplasmas are centrally important as mediators of mycoplasma-cell interactions, and as targets of the host immune response. The research effort proposed is aimed at characterizing and isolating, by immunochemical techniques, antigens of M. hyorhinis that are 1) located at the cell surface, and 2) recognized by swine during chronic mycoplasmal arthritis. Immunofluorescence, immunoelectron microscopy, and binding experiments will also be utilized to assess the interaction of mycoplasmas with the murine T-lymphoid cell surface at the molecular level.